A Boeing 737 MAX 8 airplane being built for Oman Air at the company’s Renton, Washington facility in April 2019.Photo: Ted S. Warren (AP)

At least four current or former Boeing employees called a Federal Aviation Administration hotline to report issues with the company’s 737 Max line of jets on April 5, the day after Ethiopia’s minister of transportation released a preliminary report on the crash of Ethopian Airlines flight 302 in March 2019 that killed 157 people, CNN reported this weekend.

The Ethopian crash was the second involving the 737 Max line. Lion Air Flight 610 went down in the Java Sea in October 2018, killing 189 people.

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According to CNN’s source, the calls to the FAA hotline mainly involved now widely known issues with a piece of equipment called the angle of attack sensor, which measures the angle of the plane in the air, and its Maneuvering Characteristics Augmentation System (MCAS). That system, designed to deal with potential issues that could arise from the plane’s redesigned engine placement, automatically adjusts the plane’s angle to prevent stalling.

If the angle of attack sensor malfunctions and feeds bad data into the system, a situation reportedly made worse by a decision to have MCAS only rely on one of the two sensors on board at a time, MCAS could repeatedly activate and send 737 Max jets into a nosedive—an issue which Ethiopian authorities said was likely to blame in their preliminary report. Boeing has said such a situation can be mitigated by deactivating the system, but reports have suggested that did not work before the Ethiopian crash, as the system re-engaged itself.

However, CNN reported that one of the complaints is a new issue: A “foreign object” that damaged wiring attached to the sensor. The FAA told CNN it may be opening a new investigation as a result:

The FAA tells CNN it received the four hotline submissions on April 5, and it may be opening up an entirely new investigative angle into what went wrong in the crashes of two Boeing 737 Max commercial airliners — Lion Air flight 620 in October and Ethiopian Air flight 302 in March.

Among the complaints is a previously unreported issue involving damage to the wiring of the angle of attack sensor by a foreign object, according to the source.

Boeing has reportedly had previous issues with foreign object debris in its manufacturing process; The New York Times reported metal shavings were found near wiring of Boeing 787 Dreamliner planes, and the Air Force stopped deliveries of the Boeing KC-46 tanker after foreign object debris was found in some of the planes coming off the production line.

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The entire 737 Max line is currently grounded, and the planes will not be allowed to return to service in the U.S. unless they are cleared by the FAA. Earlier this month, the FAA released a draft report stating that a planned Boeing software update for the jets is “operationally suitable,” but the groundings are expected to continue until at least late May or early June. (It’s not clear whether the allegations reported on Saturday will affect that timeline.) Boeing has also said it will make an angle of attack sensor indicator that displays the readings from both sensors side by side a standard feature of the 737 Max; that system was previously only offered as an optional upgrade.

Whistleblowers have reportedly separately claimed that FAA inspectors who approved the 737 Max line were not properly trained and did not possess the required certifications. As CNN noted, critics have targeted the FAA for allowing the jet to be sold despite the possibility of a single malfunctioning sensor triggering MCAS, suggesting that the agency has too close a relationship with Boeing.

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In a statement to CNN, the FAA wrote, “Safety is FAA’s top priority, and we have a longstanding well-established aircraft certification process that has consistently produced safe aircraft. When certifying an aircraft, we do not consider a single factor in isolation. Rather, we look at the interaction of all elements and systems, in addition to human and other external factors.”

“The single angle of attack sensor was considered in relation to a variety of other factors, specifically well-known pilot procedures that would mitigate the effects of a failure,” the FAA added. “MCAS design, certification tests, and cockpit procedures were evaluated using a standard industry approach to failure analysis.”

Boeing “did not immediately respond to a request for comment on the whistleblower reports,” CNN wrote.